The following invention relates to a communication system in which bidirectional communication is provided over a conductive link from a central controller to a remote substation wherein the central controller also provides power over the conductive link to the remote substation.
In some applications for communications systems having a central data processing controller linked by a single pair electrical conductor to a remote data processing substation, it is sometimes necessary to provide the remote substation with electrical power over the conductor on a time-shared basis with the data. This may include applications in which the remote station is movable along a track or rail where the track provides the conductive link for data transmission and reception. Since it is in motion, the remote data processing substation may not have available to it a source of power, and it is therefore necessary in such cases to provide power to the remote substation along with data over the single pair conductive link. Also, at times, the single pair conductive link may be of the type which is prone to intermittent open circuits. This can occur when the conductive link is in the form of a rail or track, or where the link must pass through a slip ring where electrical continuity is maintained by a contact bearing against the slip ring or track.
One example of an environment to which the invention relates is in a communications controller located on the steering wheel of an automobile which communicates with a central console located underneath the dashboard. Systems have been proposed for automobiles in which a communications input and display for controlling such automobile functions as a radio, tape deck or cellular radio telephone would include push buttons or contact pads to control various functions related thereto. For example, functions for a tape deck such as "play," "fast forward," or "reverse" as well as volume and tone controls have been located on the steering wheel to control the operation of a tape deck mounted in its usual location. A system generally of this type may be obtained from Blaupunkt and is known as a Blaupunkt-Rinspeed Intelligent Steering Wheel. The Blaupunkt system includes push buttons to operate an AM/FM cassette receiver, a CD player and a cellular radio telephone. In this system, however, communication from the steering wheel to the central console is via infrared links where the infrared transmitters in the wheel are powered by batteries. This is a disadvantage because of the need to change the batteries and because the batteries may run out of charge at inopportune times. This could be especially troublesome with a cellular radio telephone. Also the infrared link is one way communication only, and the remote control on the wheel is incapable of receiving data from the console.
In most automobiles, the only conductive link that extends through the steering wheel column is the horn wire with the column itself supplying the second conductive portion of the wire pair. The integrity of the horn wire is maintained by a contact and slip ring arrangement so that the steering wheel may be turned through any arc and still have the horn available. The idea of utilizing the horn link as both a communication link and a means for powering the remote data terminal has not been pursued in the past for the application described above because, given the low voltage nature of data pulses, and given the noise generated by the slip ring contact, communication from the remote data and display substation to the central data processing controller has been impractical. The slip ring contact is so noisy that the receivers at either end of the link interpret noise spikes as data pulses. Also digital communication can easily be lost, because although the slip ring contact is sufficient to maintain the integrity of the horn signal, which is a relatively high voltage and high current signal, the link would appear to be an intermittent connection to the microprocessors that would be needed at either end.
Another example of the environment in which the invention may be found is in a printer having a print head which must move along a pair of metal tracks. The print head must be powered from the metal tracks and must also receive data along this link from the printer. Another example is a model railroad. It is desirable in the case of a model railroad to power the locomotive as well as send data through the track. In the past the locomotive has not been able to send data back to the central controller, which would be a desirable feature. This data could include speed or distance travelled. The problem with attempting any more than the most elementary form of analog communication in any of these cases is the noise generated by the sliding contact.